This invention relates to a cryogenic process for obtaining oxygen of intermediate purity by a two-stage low-temperature rectification of air in two independent and distinct rectifying columns spaced apart from each other. By oxygen of intermediate purity is meant on a volume % basis a fractionated air stream containing 35 to 95%, especially about 55 to 90% oxygen.
A generally known process comprises withdrawing an oxygen-rich fraction in the liquid phase from the sump of a low-pressure column, pumping said fraction to the head of a high-pressure column, and vaporizing said fraction at that location in heat exchange with condensing nitrogen. Such a process is shown in German Pat. No. 827,364 wherein precooled air compressed to 5 bars is subjected to the first rectification stage in the high-pressure column. The oxygen-rich liquid fraction obtained in the sump of the high-pressure column is expanded via an expansion valve into the low-pressure column and subjected therein to a further rectification. The nitrogen required for this purpose is withdrawn in the gaseous phase from the head of the high-pressure column, liquefied in the condenser arranged in the sump of the low-pressure column, and fed to the head of the low-pressure column by way of an expansion valve. The condensation of the nitrogen takes place under heat exchange with the strongly enriched liquid oxygen fraction accumulating on the low-pressure side of the condenser; this latter fraction is partially revaporized during this step, but in part is also conveyed by means of a pump into the head of the high-pressure column, where it is vaporized, in turn, in a condenser under heat exchange with condensing nitrogen. This strongly enriched oxygen vapor produced in the head condenser of the high-pressure column is withdrawn, in part, as product oxygen but, in part, is recycled into the evaporation space above the condenser of the low-pressure column, to continue to take part in the rectification process.
If the aforementioned conventional process is utilized in the production of oxygen having merely an intermediate purity, there is the disadvantage that the process air must be compressed to a relatively high pressure, determined by the oxygen concentration in the product and the pressure in the sump of the low-pressure column. Consequently, the amount of power required for this purpose is relatively high as compared to the required degree of purity of the product oxygen.